Image-forming systems based on diazo compounds are widely used. Such systems can be used to form images in a variety of ways, amongst which the diazotype process and diazo lithography are of particular importance.
In the diazotype process, two components are necessary in order to produce an image, namely a photo-sensitive diazo compound (in particular a diazonium salt) and an image-forming azo coupler (for example a phenolic or amino compound). One-component materials may be prepared that contain only the diazo compound; the coupler is then added to the developing solution. Two-component materials are also available, which contain both components; these have the advantage that after image-wise exposure they can be subjected to dry development, either by a vapour (usually ammonia vapour) or by heat. Diazotype papers can be used, for example, for the direct production of a positive copy from an engineer's drawing or the like.
Diazo compounds can be used to harden a polymer imagewise; this property is employed in the production of diazo relief images (diazo lithography). Compared with dichromatesensitized colloids, the polymers sensitized with diazo compounds can have higher printing speeds and better keeping properties, and can be used for pre-sensitized printing plates.
Detailed accounts of diazo imaging systems will be found in Kosar, "Light-Sensitive Systems" (1965) published by John Wiley & Sons, and in Jacobson and Jacobson, "Imaging Systems" (1976), published by Focal Press Limited, both of which are incorporated herein by reference.
Diazo compounds in general are sensitive only to radiation in the near ultra-violet, violet and blue regions of the spectrum. The photochemical sensitivity of the diazo compounds in general use is in each case limited to a very narrow spectral region, the peak sensitivity being commonly at 375 nm, although with particular diazo compounds the peak sensitivity may be at a higher wavelength, for example from 400 to 420 nm. The sensitivity spectrum corresponds closely to the absorption spectrum when the absorption of the base material, e.g. paper, has been taken into consideration.
It is possible to sensitize diazonium compounds to decomposition by light of other wavelengths, for example red light. Thus, the absorption spectrum of a solution of 4-diazodiphenylamine sulphate in the presence of methylene blue and sodium p-toluenesulphinate exhibits a peak at about 675 nm.
It will be clear that for optimum exposure of a diazo-sensitized imaging material, it is necessary to irradiate that material with actinic radiation containing wavelengths to which the diazo compound is substantially sensitive. Suitable radiation can be produced by mercury-vapour lamps, the light output of which has peaks (the mercury lines) at 313.2, 334.2, 336, 404.7, 435.8, 546.1 and 577 nm. Of these, the two highest and the two lowest peaks are of little practical importance. The peaks at 366 and 404.7 nm have a destructive effect on a large number of benzene and naphalene diazonium salts and are therefore most important for, for example, standard diazotype papers. The peak at 435.8 nm is of particular importance with the so-called superfast diazo compounds (e.g. the morpholino derivatives of diazo-p-aminobenzenes), the absorption spectrum of which is shifted towards the visible region. Image-wise exposure of the diazo-sensitized material is effected through an original (or intermediate original) having an image (whether positive or negative) defined by areas having a differential opacity to actinic light. It will be evident that for optimum image-wise exposure, the image in the original or intermediate original should be defined by one or more areas having a high absorption (low to zero transmission) of wavelengths to which the diazo compound is substantially sensitive, the remaining area or areas of the original or intermediate original having a high transmission of such wavelengths.
As indicated above, the present invention is concerned with novel relief image-forming materials that can be used as intermediate originals in the above manner. Imaging systems based on wash-out reliefs have been known for many years and are based on the phenomenon that when certain materials or combinations of materials are applied as a coating or layer on a support and acted upon by light either a hardening or a solubilising effect takes place in the light-exposed areas. In the case of the hardening effect by light the unexposed areas can be washed away with water or other solvent resulting in a positive image from a negative original; in the case of the solubilising effect by light the exposed area can be washed away with water or other solvent, resulting in a positive image from a positive original.
Other systems have been proposed in which the light-exposed areas of the layer can be insolubilised to water and solubilised to another solvent. Thus, by the correct choice of wash-off liquid, a negative or a positive image can be obtained from the same original.
In most cases the exposed areas do not substantially differ visually from the unexposed areas but only physically and/or chemically, especially in the degree of solubility. The exposed layers are then used to produce visible images.
This can be done in various ways. Thus, for example:
(a) a dye or pigment can be introduced into an exposed dichromated colloid layer by imbibition into the relief followed by rinsing with water or other solvent whereby the soluble areas of the layer are washed away leaving a visible relief image; or
(b) silver halide can be included in a dichromated colloid layer, whereby the silver halide is retained in the hardened layer after exposure and washing and the visible silver image can be obtained by development in a photographic developer followed by washing; or
(c) a dye or pigment can be introduced into the layer before coating whereby, after exposure, the soluble areas of the coating can be washed away together with the dye or pigment leaving a visible relief image.
There are drawbacks to these methods, however. Method (a) involves the use of inks and dyes after exposure and washing, which is messy and tedious as well as requiring an extra operation. Method (b) involves expensive silver salts and the extra operation of development of the silver image and further washing.
Method (c) employs the principle of the "pigment paper process" (carbon printing) first proposed in 1855 by the French engineer and chemist Alphons Louis Pointevin, who added powdered carbon to a sensitized gelatin solution thereby making an advance in the direction of obtaining visible and permanent images. After exposure and washing, the carbon particles remained imbedded in the tanned gelatin. But, as stated by Kosar, op cit page 116: "The use of silver halides is preferred over the incorporation of pigments such as carbon black or dyes because such substances would absorb a large proportion of the light required for the photochemical reaction. This would necessitate greatly increased exposure times." Nevertheless method (c) is greatly preferred over methods (a) and (b) for its easier processing, involving, as it does, simple washing and drying, and for its avoidance of the use of silver salts (which are expensive).
The light-absorption properties of pigments or dyes incorporated in any relief photo-imaging system have hitherto been a barrier to the obtaining of reasonable exposure times for such systems using conventional exposure or printing equipment. There is thus a need for a relief photo-imaging system that includes a suitable colouring agent whilst exhibiting acceptable exposure times.
The production of a visible image upon exposure of a pre-sensitized lithographic plate may be desirable in that it permits an easier assessment for quality. Moreover, in the "step and repeat" process of preparing a lithographic plate, such a visible image allows the operator to ascertain at any stage of the process which areas of the plate have already been exposed. Thus, GB-PS No. 1,041,463 discloses a lithographic plate comprising a metal sheet and a layer thereon of a light-sensitive composition, which composition comprises a diazo compound and an acid-base indicator. Exposure of the composition to light causes the indicator to change colour owing to the release of an acid upon the decomposition of the diazo compound under the action of light.
The preferred colour change described in GB-PS No. 1,041,463 is from colourless to a distinctly visible but ultraviolet transmitting colour; clearly, the UV-transmitting properties of such an indicator would be of little utility in an intermediate original, as discussed above, which would require one or more areas of high actinic opacity. The said British specification does contemplate the use of indicators that act as ultra-violet absorbers before or after exposure. This means that absorption must also occur during exposure, which can only serve to prolong the exposure time.
GB-PS No. 1,386,586 discloses a photo-resist composition comprising a light-modifiable material, a photosensitizer and a dye in an Homolka base form. Upon image-wise exposure to light the light-struck areas are modified to permit selective removal of material from the exposed or the unexposed areas of the layer in order to form a relief image. It is desirable that the image should be readily visible. To this end, the Homolka base is converted into the parent dyestuff either during exposure, by incorporating a material into the composition that will generate an acid or acidic compound on exposure to actinic light, or during development by carrying out the selective removal of material by treating the layer with a solvent that contains an acid or acidic compound.
The use of an Homolka base necessitates the use of a non-acidic composition. Furthermore, there is no teaching in GB-PS No. 1,386,586 with regard to the formation of relief images that would be suitable for use as intermediate originals.